For some time, plastic materials have been successfully cast into single vision lenses, usually in some definite diameter and thickness. Such castings are referred to as uncuts. These are finished by edging to size and shape in a prescription laboratory.
When the uncuts are obtained by grinding and polishing from semifinished plastic castings, i.e., castings requiring grinding and polishing on one side, inferior lenses result due to the physical instability of the casting during processing. Plastic does not polish as readily as glass and many steps of processing are required to get a satisfactory optical surface, the final step usually being performed with a velour type pad. The laboratory thus experiences high costs and the patient gets inferior lenses. As a result most single vision plastic lenses are provided in the uncut cast form, requiring only edging to size and shape.
When it is desired, using known techniques, to provide bifocals made out of plastic, the requirement of the bifocal addition makes the ability to supply uncuts as stock items virtually impossible because of the millions of combinations necessary to meet prescriptive requirements. Heretofore, when a plastic bifocal lens was desired, one procedure was to supply the optician with semifinished blanks from which he ground and polished the uncuts with the inferior results and high costs referred to. A satisfactory method of casting uncut bifocals out of plastic which would require no additional grinding or polishing of the optical surfaces, has, therefore, long been desirable.
One of the known techniques of finish casting multifocal plastic lenses is described in U.S. Pat. No. 3,056,166 to Weinberg. It discloses the making of a finished cast plastic lens by using a pair of mold parts coupled together by an annular gasket. When a bifocal addition is reqquired, Weinberg states that a mold part containing the proper bifocal addition at the proper location in the surface thereof is selected as one of the mold parts, and that a corresponding gasket is also selected. Where it is desired to introduce corrective prism into the lens, Weinberg states that it is necessary to cut the gasket by a specially constructed device. Thus, using the techniques described by Weinberg for the direct casting of finished bifocal plastic lenses is cumbersome and uneconomic as it requires carrying an inventory of a very large number of bifocal molds and gaskets and also requires operating a special machine tool at or near the casting station.
It is therefore an object of the present invention to provide a means for the inexpensive casting of high quality multifocal lenses from plastic material, wherein no further grinding or polishing operations of the optical surfaces are required.
It is a further object of the invention to provide a means for providing such lenses using a limited and manageable number of mold and gasket parts.
It is a still further object of the present invention to provide a simplified and inexpensive means to position the bifocal addition a predetermined distance from the optical center of the distance field, as may be required by the prescription involved.
It is another object of the present invention to provide a simple and inexpensive means of introducing corrective prism into a cast multifocal lens.